Sublimation type thermal transfer methods have been widely used to form a thermal transferred image on a transfer receiving article because such methods can easily form images with excellent transparency and high reproducibility of neutral tints and high gradation, as well as high quality equivalent to conventional full-color photographic images. As prints in which a thermal transferred image is formed on a transfer receiving article, digital photographs, and identification (ID) cards, which are used in various fields such as identification papers, driver's license cards, membership cards, are known.
For forming a thermal transferred image in accordance with the sublimation type thermal transfer method, a thermal transfer sheet having a dye layer provided on one surface of a substrate and a transfer receiving article, for example, a thermal transfer image-receiving sheet having a receiving layer provided on one surface of another substrate, are used. Then, the dye layer of the thermal transfer sheet is superposed on the transfer receiving article, and heat is applied by a thermal head from the back side of the thermal transfer sheet to allow the dye in the dye layer to migrate. As such, a print in which a thermal transferred image is formed on the transfer receiving article can be obtained. With respect to the sublimation type thermal transfer method, since the migration amount of dye can be controlled by the amount of energy applied to the thermal transfer sheet, it is possible to perform a density gradation. Thus, this method can form a high-quality print comparable to full-color photograph images, being very vivid and excelling in transparency, color reproducibility of halftones and gradation.
Recently, in order to meet demands to obtain prints in which a thermal transferred image is formed on any transfer receiving article other than a thermal transfer image-receiving sheet, an intermediate transfer medium in which a receiving layer is releasably provided on a substrate has been proposed (e.g., Patent Literature 1). According to this intermediate transfer medium, the dye in the dye layer of the thermal transfer sheet is allowed to transfer onto the receiving layer of the intermediate transfer medium to form a thermal transferred image. Thereafter, it is possible to transfer the receiving layer onto any transfer receiving article by heating the back side of the intermediate transfer medium, thereby obtaining a print in which the thermal transferred image is formed on any transfer receiving article.
In a print obtained by forming a thermal transferred image on the receiving layer of a thermal transfer image-receiving sheet in accordance with the sublimation type thermal transfer method or in a print obtained by forming a thermal transferred image on the receiving layer of an intermediate transfer medium in accordance with the sublimation type thermal transfer method and retransferring this receiving layer onto any transfer receiving article, the receiving layer on which the thermal transferred image is formed is located on the outermost surface of the print. However, although the thermal transfer image formed on the receiving layer in accordance with the sublimation type thermal transfer method is excellent in the formation of a gradation image as mentioned above, the print formed has a disadvantage such as inferior physical durability including abrasion resistance because the print, unlike prints formed with usual printing ink, is formed not with pigments but with dyes each having a relatively low molecular weight, and includes no vehicle.
Under such circumstances, a wide range of attempts have been made, in which a protective layer transfer sheet having a substrate and a protective layer (it may be referred to as an exfoliate layer or transfer layer) provided releasably from the substrate is used to transfer the protective layer onto a receiving layer having a thermal transferred image formed thereon (e.g., Patent Literature 2). According to a protective layer transfer sheet as that proposed in Patent Literature 2, transferring the protective layer onto the receiving layer on which the thermal transferred image is formed can improve the durability of the thermal transferred image.
Alternatively, as a variation of the protective layer transfer sheet provided with the protective layer, a thermal transfer sheet in which a dye layer of at least one color and a protective layer releasable from the substrate are provided on one surface of the substrate, as being frame sequentially, has been proposed (e.g., Patent Literature 3). According to the thermal transfer sheet as that proposed in Patent Literature 3, it is possible to form a thermal transferred image and transfer a protective layer onto the thermal transferred image formed by using one thermal transfer sheet.
By the way, a thermal transfer sheet is usually stored and used in a rolled-up state. When the thermal transfer sheet is in the rolled-up state, the dye layer is in direct contact with the back side of the thermal transfer sheet (hereinafter, it is referred to as a back face layer). Thus, in the case where the dye of the dye layer is likely to migrate to the back face layer side, the dye contained in the dye layer migrates (so-called “is kicked”) to the back face layer when the thermal transfer sheet is rolled up. Alternatively, in the case where a thermal transfer sheet in which a dye layer and a protective layer releasable from the substrate are provided on one surface side of the substrate, as being frame sequentially, is employed as described above, if the dye that has been kicked to the back face layer remigrates (so-called “backs”) to the protective layer, various problems may occur, such as deterioration in the quality of the print onto which the protective layer has been transferred and a decrease in the transferability of the protective layer.
Additionally, a protective layer to be transferred onto the thermal transferred image is required to have high light fastness, in addition to physical durability such as abrasion resistance.